The present invention relates generally to resonant inverters, such as those used in an electronic ballast to provide power to a gas discharge lamp. More particularly, the present invention pertains to methods of operating a resonant inverter-type electronic ballast to provide stable operation of, and to control power to, a gas discharge lamp.
Electronic ballasts are commonly used to power gas discharge lamps, such as a fluorescent lamp. A typical electronic ballast will include a half-bridge resonant inverter driven by an inverter drive circuit. The inverter drive circuit controls the switching of the top and bottom inverter switches so that the inverter operates at or near the self-resonant frequency of the inverter. Using a driven resonant inverter allows for the lamp power to be controlled (for dimming and/or stable power regulation) by varying the inverter frequency and/or by varying the pulse width of the inverter output.
Stable operation of the lamp is important in the design and operation of an electronic ballast. For many applications, an ideal resonant inverter would act as an ideal current source in which the open loop output impedance is infinite so that all of the current generated by the inverter flows to the lamp. Also, to achieve optimal efficiency while lowering component stresses, a preferred resonant inverter design would insure zero-voltage switching of the inverter transistors.
Unfortunately, prior art resonant inverters that operate as current sources suffer from several problems caused by deficiencies in the inverter switching methods. For example, use of frequency modulation to control the inverter output does not guarantee zero voltage switching. Moreover, with frequency modulation, the higher frequencies and higher lamp voltages associated with lamp dimming create undesirable circulating currents through the component and parasitic capacitances in the circuit. Resonant inverters that use symmetric pulse width modulation (also known as dead time modulation) also have a high likelihood of non-zero voltage switching.
What is needed, then, is a method of operating a resonant inverter that maximizes the open loop output impedance by operating the inverter close to self-resonance and that guarantees zero voltage switching of the inverter transistors.